Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various layers using lithography to form circuit components and elements thereon.
A transistor is an element that is utilized extensively in semiconductor devices. There may be millions of transistors on a single integrated circuit (IC), for example. A common type of transistor used in semiconductor device fabrication is a metal oxide semiconductor field effect transistor (MOSFET). Conventional MOSFETs have one gate electrode that controls a channel region, and are often referred to as single gate transistors. Early MOSFET processes used one type of doping to create single transistors that comprised either positive or negative channel transistors. Other more recent designs, referred to as complementary MOS (CMOS) devices, use both positive and negative channel devices, e.g., a positive channel metal oxide semiconductor (PMOS) transistor and a negative channel metal oxide semiconductor (NMOS) transistor, in complementary configurations.
Conventional bulk single-gate planar MOSFET devices cannot achieve the requested performance for future technology nodes of 45 nm or beyond. The classic bulk device concept is based on a complex three-dimensional doping profile, which includes channel implantation, source and drain region implantation, lightly doped drain (LDD) extension implantation, and pocket/halo implantation processes, which are not further scalable down in size, because of an increase in dopant fluctuations and stronger parasitic short channel effects, due to lack of potential control in the channel region and the deep substrate. Therefore, the ITRS Roadmap, e.g., disclosed in the 2002 edition of International Technology Roadmap for Semiconductors (ITRS), which is incorporated herein by reference, has proposed two novel design concepts: a fully depleted planar silicon-on-insulator (SOI) MOSFET device, and a vertical multiple-gate finFET (fin field effect transistor) or tri-gate device.
Thus, transistors with multiple gates are an emerging transistor technology. A double gate transistor has two parallel gates that face each other and control the same channel region. A finFET is a vertical double gate device, wherein the channel comprises a vertical fin comprising a semiconductor material, typically formed on a silicon-on-insulator (SOI) substrate. The two gates of a finFET are formed on opposing sidewalls of the vertical fin. A tri-gate transistor has three gates that control the same channel region, e.g., the channel comprises the vertical fin, two of the gates are formed on the sides of the vertical fin, and a third gate is formed on the top of the fin. A finFET structure is similar to a tri-gate transistor, with the third gate being blocked by an insulating material or hard mask disposed on top of the fin. FinFETs and tri-gate transistors, and some of the manufacturing challenges of forming them, are described in a paper entitled, “Turning Silicon on its Edge: Overcoming Silicon Scaling Barriers with Double-Gate and FinFET Technology,” by Nowak, E. J., et al., in IEEE Circuits & Devices Magazine, January/February 2004, pp. 20-31, IEEE, which is incorporated herein by reference.
FinFETs and tri-gate transistors may be used to form CMOS devices. One or more finFETs can be used as a PMOS and/or NMOS transistor: often, two or more fins in parallel are used to form a single PMOS or NMOS transistor. FinFETs can be scaled or reduced in size more aggressively than planar transistor structures, and show lower gate-induced drain leakage (GIDL) current, as described in a paper entitled, “Extremely Scaled Silicon Nano-CMOS Devices,” by Chang, L., et al., in Proceedings of the IEEE, November 2003, Vol. 91, No. 11, pp. 1860-1873, IEEE, which is incorporated herein by reference. However, multiple gate transistors such as finFETs are more difficult and complicated to manufacture than planar CMOS devices, and they require distinctly different materials and introduce a variety of processing challenges.
Furthermore, it is important to design CMOS devices so that a symmetric threshold voltage Vt for the NMOS and PMOS transistors of the CMOS device is achieved. However, it is difficult to find materials, device structures, and manufacturing processes that will achieve a symmetric threshold voltage Vt as devices are made smaller, and particularly for advanced transistor designs having multiple gates.
Thus, what are needed in the art are improved structures and manufacturing processes for multiple gate transistors.